The present invention relates to a method for treating industrial or sanitary waste sludge having both solid and liquid components. The present invention is, more specifically, disclosed in the context of its preferred embodiment, which relates to a method for processing waste paint sludge from a painting operation, to dry it, remove volatile organic compounds, substantially reduce its weight and volume and, ultimately (in some cases), to produce a recyclable product.
The disclosures of U.S. Pat. Nos. 4,980,030, 4,750,274, and 4,222,319, as well as Haden's copending patent application, Ser. No. 08/371,358, titled "Method for Drying Paint Sludge and Raw Paint," filed on Jan. 11, 1995, are incorporated by reference into this application.
Recently, increasing environmental concerns and federal regulations have sought to limit the type and quantity of waste generated by various industries, including sanitary and industrial waste sludge, as well as sludge generated by the automotive paint industry. In response to this problem, manufacturers have developed various methods for treating sludge, including automotive paint sludge, and creating useful byproducts from it.
When an automobile is painted in a paint booth, the excess paint solids are collected beneath the paint booth, typically in a wet booth or a water-wash system. The material recovered from such processes, either by chemical separation, membrane technology, filtration, or scrubbing, is known as paint sludge.
It will be appreciated by those skilled in the art that the waste paint sludge treated in accordance with the present invention is a complex material and is different from most other waste sludges. First, such sludge includes volatile organic compounds ("VOC's"), such as thinners, solvents and detackifying agents. Second, such sludge includes uncured polymeric paint resins that can "cure" or cross-link upon the heating and volatilization of the constituent liquid components. Thus, paint sludges can become very sticky when dried. Third, paint sludges can include inorganic pigments and heavy metals. These components are often hazardous and/or toxic.
Disposal of paint sludge is a problem of considerable complexity that faces paint booth operators. Currently available disposal technology is based upon the principles of incineration, chemical and physical treatment, and solidification, as discussed in U.S. Pat. Nos. 4,980,030 and 4,750,274.
Prior art methods for drying paint sludge have relied on traditional heating methods applied to a mixing machine. The sludge is stirred constantly and heated until all of the water initially present in the sludge is driven off. The remaining paint solids can then be heated further to cure the remaining paint resins, with the object of providing a dry powder. Two types of machines are known to perform this heating process. One, the assignee's own DRYPURE.RTM. drier, is a hollow flight screw mixer heated with hot oil, the use of which is described in U.S. Pat. No. 4,750,274. Another known commercial system, sold by Salem Engelhardt, utilizes a rotating tray dryer, which is believed to be heated by natural gas.
While DRYPURE.RTM. has met with some commercial success, both types of machines are susceptible to the operational problem that, after the water is removed, the paint sludge can solidify into a single hard mass that can lock up the machine. Removal of the hardened material is difficult and time consuming. The inventor has discovered that the solidification problem can be avoided to some extent by chemical pretreatment of the sludge, as disclosed in Haden's copending patent application, Ser. No. 08/371,358, titled "Method for Chemically Treating Paint and Paint Sludge. "
Another problem with these known thermal drying techniques is that they require a great deal of energy. Most of the energy used to dry the sludge is required to boil off the water in the sludge. Since each pound of water, once it first reaches 212.degree. F., requires an additional approximately one thousand BTUs to be boiled off, and paint sludge typically contains about 75% water, those in the art will readily appreciate that a significant amount of energy is required to vaporize the water within the paint sludge.
A further problem with known thermal techniques for drying paint sludge is that, as the percentage of sludge solids increases, it becomes increasingly more difficult to remove water from the sludge. Conventional thermal techniques dry sludge by heat conduction, and sludge solids tend to inhibit this heating process as their relative concentration increases. This is due to the fact that sludge is a good insulator or, conversely, a poor heat conductor. Therefore, with conventional thermal dryers transferring heat into the sludge becomes progressively more difficult as the drying process continues.
A further problem with known thermal techniques is that these systems require significant warm-up times (up to two hours) and corresponding cool-down times, due to their larger thermal mass. For example, with the screw mixer disclosed in U.S. Pat. No. 4,750,274, the "thermal mass," (i.e., the mass that must be heated to heat the sludge), includes a good deal of mass unrelated to the sludge, including the oil tanks, piping, jackets containing oil, etc.
The present inventor has found, through experimentation, that when sludge is heated, preferably up to about 212.degree. F., by any means, most of the water within the sludge is easily released by mechanical dewatering. For example, the inventor has found that for sludge initially including 23% solids, about two-thirds of the water drains off readily after heating the sludge to about 212.degree. F. using only gravity to induce the draining. As those of skill in the art will readily appreciate, draining the sludge liquid is far more economical, in terms of power consumption, than boiling it off from the sludge solids. For example, heating one pound of water from 70.degree. F. to 212.degree. F., without vaporization, requires about 142 BTUS. However, due to the latent heat of vaporization, heating one pound of water from 70.degree. F. to 212.degree. F., and then vaporizing the water, requires about 1112 BTUs--nearly nine times the energy.
The present inventor has also found that, contrary to known heating methods, the sludge should not be stirred during heating. The less stirring or agitation the paint sludge undergoes as it dries, the less likely it is to solidify or "go plastic." Further, it has been found that no mixing or stirring during heating yields the best drying results, regardless of the heating method employed. The inventor believes that the reason for this is that the shear force exerted on the detackifiers present in the paint sludge during stirring displaces the detackifiers and exposes the raw paint particles. Continued stirring is then thought to encourage conglomeration of the naturally sticky paint particles.
Another disadvantage of known sludge processing techniques is that they typically operate in a "continuous flow" manner, so that a stoppage of one operation locks up the entire system. When sludge "plasticizes," it will be appreciated that the "continuous flow" system can be a serious disadvantage.
A further problem associated with known thermal techniques is their relatively high operational and maintenance costs.
Accordingly, an improved thermal technique for drying paint sludge in an economic manner is needed.